1. Field of the Invention
The present invention relates to a cutting tool having a tool main body formed of ceramic which can be used as a throw-away tip, an end mill, a drill, or the like, and more particularly, to a cutting tool in which a surface coating is provided on the tool main body formed of ceramic to improve wear resistance.
2. Description of the Related Art
Conventionally, hard materials, such as carburized and quenched steel, die steel and tool steel, have been ground by use of a grinding stone. However, grinding involves a problem of low machining efficiency. Therefore, in order to perform machining at higher speeds, attempts have been made to replace grinding with cutting using a cBN (cubic Boron Nitride) tool or a ceramic tool formed of an alumina-titanium carbide composite ceramic. Recently, cBN tools have often been used, because a ceramic tool formed of alumina-titanium carbide composite ceramic has a short service life and poor reliability, and cannot withstand a further increase in cutting speed.
Although cBN tools provide excellent cutting performance, they are very expensive, because cBN tools are fabricated by utilizing a super-high pressure technique as in the case of artificial diamond, thereby hindering the popularization of cBN tools. In view of the foregoing, various ceramic tools have been proposed which are less expensive and provide performance comparable to that of cBN tools in high speed cutting. For example, Japanese Patent Application Laid-Open (kokai) Nos. 4-289002, 5-69205, and 7-136810 disclose techniques for improving wear resistance and fracture resistance of a ceramic tool by forming a coating layer of titanium carbide or titanium nitride on the surface of the ceramic base member by CVD (Chemical Vapor Deposition).
However, in the techniques disclosed in the above patent publications, since inadequate consideration is given to residual stress produced in the coating film at the time of its formation, great improvement in performance cannot be expected. That is, in CVD, a coating-layer forming substance is deposited on the surface of a ceramic base member, while being synthesized through chemical reaction of a gaseous source material. This reaction is typically performed at a high temperature of 1000° C. or more in order to activate the source material. When the difference in coefficient of thermal expansion between a ceramic base member and a coating layer formed thereon is large, a large residual stress is produced in the coating layer when the formed layer is cooled to room temperature. Moreover, when the coating layer is composed of a plurality of sub-layers of different materials having different coefficients of thermal expansion, the differences in coefficient of thermal expansion among the sub-layers cause generation of residual stress. Although ceramic tools have excellent wear resistance as compared with tools formed of high-speed tool steel or cemented carbide alloy, they have insufficient fracture resistance, and particularly insufficient resistance to flaking fracture (flaking of a cutting edge to a shell-like shape) which occurs during machining of the above-mentioned hard materials to shorten the service life thereof. Therefore, even when the above-mentioned coating layer is provided, satisfactory performance in terms of fracture resistance cannot be expected if a large residual stress is produced in the coating layer.
Moreover, the problem of residual stress arises not only during formation of a coating layer but also during actual machining. Since a tool is heated to high temperature due to contact friction between the tool and a workpiece during actual machining, when the tool is used repeatedly, thermal shock repeatedly acts on the tool due to the repeated cycles of heating and cooling. In a typical case in which a coating layer of titanium nitride is formed on the surface of the above-described composite ceramic base member, the coating layer has a coefficient of thermal expansion higher than that of the base member. As a result, tensile residual stress is produced in the coating layer during cooling which is performed after forming the layer by CVD, which tensile residual stress also lowers the fracture resistance of the tool. In particular, since the inclination toward labor saving and high-efficient machining has become stronger in recent years, tools which can be used in cutting conditions under which large thermal and mechanical loads act on tools have been developed. However, the above-described problem has been an obstacle in such development.